Winning metals has been a human activity since time immemorial. Civilization has grown with this art; and it is safe to say that the production of metals is the genesis and sustenance of many aspects of modern technology. At the present time, mankind utilizes metals at a large and rapidly increasing rate. For this reason, improvements in techniques for obtaining metals have immediate interest.
The above facts are adequately illustrated by the history of copper. Mankind emerged from the Stone Age upon discovery of copper in its native form. The dawn of the Bronze Age was circa 8000 B.C. -- when it was discovered that this copper-tin alloy could be readily shaped into implements and weapons. Copper deposits on Cyprus were worked as early as 3000 B.C. by the Egyptians and these deposits became the chief source of the metal for the Roman Empire. In 1556, Agricola recorded the history of copper. In 1963, world refined copper output exceeded 3,800,000 short tons.
Nickel was isolated by Cronstedt in 1751. By 1804, the properties of the pure metal were known with reasonable accuracy.
Referring to the section on copper in Kirk-Othmer Encyclopedia of Chemical Technology, second edition, volumn 6, page 131, and The Winning of Nickel, by Boldt, Jr., et al., D. Van Nostrand Co., New York, N.Y. (1967), the production of copper and nickel from ores are tedious, complex processes. Clearly, commerce could not bear the cost of such multi-step processes if these metals were not so important. A detailed discussion of all ramifications of art-known methods for the production of copper and nickel would be out of place here. It is sufficient to relate the following facts.
Copper and nickel are both present in some ores worked today. Since 1899 nickel has been refined by the Mond process which comprises reacting nickel with carbon monoxide to form nickel carbonyl and subsequent decomposition of this product to carbon monoxide and nickel. In the section on nickel in KirkOthmer (supra), second edition, volume 13, page 735, (739) there is described a hydrometallurgical refining process for nickel (practiced by Sherritt Gordon Mines Limited of Toronto, Canada). In this process, concentrates of pentlandite, (Ni, Fe) .sub.9 S.sub.8, are dissolved in an aerated ammoniacal solution. The nickel, copper and cobalt sulfides dissolve as ammines, with iron remaining in the residue as hydrated ferric oxide. Subsequently, copper is precipitated, and the remaining nickel solution is oxidized to destroy sulfamate. The resultant solution is treated with hydrogen at 35 atmospheres and 190.degree.C. to yield 99.9% nickel which is sintered into briquettes.
The Sherritt Gordon process is described in more detail in Boldt, Jr. (supra), page 299 ff. As described therein, the copper is removed from the ammoniacal solution by boiling off ammonia to precipitate cupric sulfide. The last traces of copper are removed by adding H.sub.2 S. This must be done before nickel is precipitated with hydrogen, to avoid contamination of the nickel with copper.
In general, nickel and associated metals are recovered and separated by pyrometallurgical, hydrometallurgical, or electrolytic refining techniques. For sulfide ores, the general operations of roasting, smelting, and converting produce a nickel matte product which is suitable for refining to pure metal electrolytically. In contrast to sulfide ore processing, the oxide ores may be more economically processed by hydrometallurgical or carbonyl processes to produce very high purity nickel. However, this is not true in all cases since one commercial operation utilizes pyrometallurgical techniques to prepare ferronickel from a laterite ore. A roasting operation decreases the sulfur content of a sulfide ore concentrate by about one-half. Previous processes have used multiple hearth furnaces, sintering machines, or fluidized bed reactors. When followed by smelting operations using shaft furnaces, reverberatory furnaces or electric arc furnaces to slag off siliceous and other oxide compounds, a typical nickel sulfide matte containing about 15% nickel-copper, 50% iron, and 25% sulfur is produced. This nickel-sulfide matte is then charged to a converter and air is blown through the charge to oxidize iron sulfide selectively. Usually, horizontal converters are used. Recently a process using a top-blown rotary converter in which an oxygen lance is blown onto the surface of the molten charge has been placed in successful commerical operation. The nickel sulfide matte essentially free of iron-produced in the converting operation, typically contains about 48% nickel, 27% copper, 22% sulfur, and less than 1% iron. This matte is sulfur deficient and, therefore, contains a metallic phase which must be processed to separate the nickel sulfide and copper sulfide. In one process, a slow cooling step is used whereby the sulfur deficient matte cast from the converter is cooled slowly over a period of several days. The nickel sulfide, copper sulfide, and copper-nickel metallics separate, allowing regular ore dressing operations to be used to separate the solidified matte into its components. Thus, the nickel-copper alloy is removed magnetically and the nickel and copper sulfides are then separated by flotation. The nickel sulfide can then be either sintered to provide 90% nickel for direct use by steel producers, or roasted to the oxide, smelted and cast into anodes for electro-refining. Alternatively, the nickel sulfide matte can be cast directly into anodes for electro-refining.
In electrolytic refining, metallic nickel of high purity is produced. A major portion of the world's nickel production includes this process as a last step in winning nickel. In addition, the recovery of precious metals and other elements such as cobalt is practiced. A divided electrolytic cell with a porous diaphragm separating the anode and cathode is used in the electrolytic process. The diaphragm prevents impure anolyte from directly contacting the nickel cathode starting sheet. The impure anolyte obtained by solution of the anode is pumped away from the cell to another area where impurities are removed. The nickel cathodes containing 99.9.sup.+% nickel are removed after about ten days operation of the cell.
In addition to the Sherritt Gordon process described above, other hydrometallurgical refining processes are commercially employed to win nickel by gaseous reduction of nickel salt solutions derived from both sulfide and oxide ores. Preparatory ore-dressing treatments provide a uniform feed for the reduction and leaching process. Leaching procedures vary depending on the particular ore treated. However, the nickel carbonates produced from the aqueous solution are calcined to marketable nickel oxide, or further sintered to upgrade the nickel metal content to about 88%. A process to recover nickel and cobalt from a limonitictype laterite ore from Cuba treated the ore with sulfuric acid at elevated temperature and pressure to dissolve nickel and cobalt preferentially. The iron remained essentially undissolved. The liquid separated from the residue contained 95% of the nickel found in the ore. After further purification of the aqueous phase, the nickel sulfate is reacted with hydrogen at high pressure and at about 190.degree.C. to recover most of the nickel as a 99.8% pure product.
Nickel is also produced in the form of ferronickel and nickel rondelles. Ferronickel is produced by a pyrometallurgical process of melting, reduction, and refining. One commercial process in New Caledonia reduces the ore with coke in electric furnaces. Another commercial process in Oregon involves mixing molten ore with ferrosilicon and crude ferronickel. By pouring the molten materials back and forth in special ladles, the molten ore is reduced and the resulting ferronickel product contains about 48% nickel. This crude product is further refined to lower the impurity level. Nickel rondelles are produced by reacting the ore with coke and gypsum in blast furnaces, blowing the nickel-iron matte and a siliceous flux with air and converting to produce a low-sulfur nickel matte, roasting to the oxide, grinding, compacting, and reducing to the metal with charcoal. The resulting nickel rondelles contain 99% nickel.
Data on the reduction of copper (II) salts with carbon monoxide has been published; Byerley et al., Met. Soc. Conf., 24, 183 (1963); Chem. Abs. 64, 13441 h (1966). Conversion of aqueous nickel to nickel carbonyl has been disclosed in Chem. Abs. 53, 12606 H (1959).